This invention relates to an ultraviolet radiation curable protective coating composition. More particularly, it relates to a silicone coating composition which, when applied to a substrate, forms a protective, abrasion-resistant coating thereon.
Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses, such as for eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering and lighter than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, such as dust, cleaning equipment and/or ordinary weathering. Continuous scratching and marring results in impaired visibility and poor aesthetics, and often times requires replacement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, good dimesional stability, as well as being self-extinguishing, and is easily fabricated. Acrylics, such as polymethylmethacrylate, are also widely used transparent plastics for glazing.
Attempts have been made to improve the abrasion-resistance of transparent plastics. For example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Pat. Nos. 3,708,225, 3,986,997 and 3,976,497, for example, describe such compositions.
In copending U.S. Application Ser. No. 964,910 coating compositions having improved resistance to moisture and humidity and ultraviolet light are disclosed. It was discovered therein that, in direct contrast to the teachings of U.S. Pat. No. 3,986,997, compositions having a basic pH, i.e., 7.1-7.8, do not immediately gel but in fact provide excellent abrasion-resistant coatings on solid substrates.
The present invention offers a significant advantage over many of the heretofore known coating compositions in that it does not require heat in order to initiate the cure reaction. The radiation cure system of the present invention expends considerably less thermal energy than conventional heat cure systems.
Although it has been known that colloidal silica can be incorporated into heat cured coating systems, the present invention demonstrates for the first time that such colloidal silicas can be utilized to improve the abrasion resistance of ultraviolet light curable coatings. These results are accomplished through the specific combination of colloidal silica with acryloxy functional silanes or water miscible hydroxy acrylates, or preferably both, with catalytic amounts of UV sensitive photoinitiators.
Prior to the present invention it had not been known to so combine colloidal silica with various acryloxy functional materials in a UV system.
In copending application Ser. No. 129,297, the present applicant has disclosed a radiation curable hardcoating composition which requires the use of the acid hydrolysis product of an alkoxy functional silane. Applicant's present invention, however, provides UV-curable coatings which possess even greater abrasion resistance and yet do not require the use of any such alkoxy silanes.
Ultraviolet light is one of the most widely used types of radiation because of its relatively low cost, ease of maintenance, and low potential hazard to industrial users. Rapid photo-induced polymerizations utilizing UV light rather than thermal energy for the curing of hard coatings offer several other significant advantages. First, faster curing coatings offer substantial economic benefits. Furthermore, heat sensitive materials can be safely coated and cured with UV light without the use of thermal energy which could damage the substrate. Additionally, the essentially solvent free media reduces the necessity for expensive and time consuming pollution abatement procedures.
Accordingly, it is one object of the present invention to provide a novel protective coating for solid substrates.
Another object of this invention is to provide a radiation curable coating for solid substrates which, when applied to the substrate, provides an improved abrasion-resistant surface thereto.
Still another object of the present invention is to provide a silicone coating composition especially well-suited for providing an improved abrasion-resistant surface to transparent substrates without impairing the optical clarity of the substrate.
A further object of the present invention is to provide a coating composition which results in an improved abrasion-resistant surface when applied to metals and metallized surfaces.